Method and means for compensating birefringence in laser systems

ABSTRACT

Birefringence induced by nonuniform heating in a laser rod is compensated for by introducing a compensating material of similar physical properties to the rod and artificially inducing birefringence in the compensating material in a sense opposite to that taking place in the laser rod.

United States Patent Koechner 1 May 9, 1972 METHOD AND MEANS FOR [56]References Cited COMPENSATING BIREFRKNGENCE IN UNITED STATES PATENTS HLASER SYSTEMS 3,484.7]4 12/1969 Koester et al ..331/94.s v [72]inventor: Walter Koechner, Malibu, Calif.

Primary Examiner-Ronald L. Wibert [73] Asslgnee. Union CarbideCorporation, New York. Assistant ammmer comad Clark 1 Attorney-Pastonza& Kelly [22] Filed: Feb. 11, 1970 [21 Appl. No.: 10,513 ABSTRACTBirefringence induced by nonuniform heating in a laser rod is [52] [1,5,("I NI/94,5 compensated for by introducing a compensating material of [51] int. Cl. Hols 3/00 similar physical properties to the rod andartificially inducing 58] Field of Search .33 1/945 birefringence in thecompensating material in a sense opposite to that taking place in thelaser rod.

wm mwsm METHOD AND MEANS FOR COMPENSA'I'ING BIREFRINGENCE IN LASERSYSTEMS This invention relates to laser systems and more particularly toa method and means for compensating birefringence induced by thermalstresses in a laser rod.

BACKGROUND OF THE INVENTION Birefringence, or the splitting of a beam ofincident light into two components which travel at different velocities,oRen is present in optical devices subject to nonuniform stress. Solidstate laser materials such as rods operating in either the steady stateor continuous wave mode of operation must dissipate an appreciableamount of heat. In cylindrical geometries which are most commonly used,the heat is removed from the circumferential surface of the cylinderthereby generating a radial thermal gradient. The varying temperaturewithin the laser rod generates nonuniform stress which in turn inducesbirefringence.

There are a large number of laser systems in which a linear polarizedbeam is required. For example, systems utilizing electro-opticalswitching, acoustooptical modulation, and frequency doubling all requirea linear polarized beam. Birefringence severely decreases the outputpower from such a laser whose beam has to be linearly polarized.

BRIEF DESCRIPTION OF THE PRESENT INVENTION In accord with the method andmeans of the present invention, birefringence induced by thermal stressin a laser material is compensated for by introducing a compensatingmaterial of physical properties similar to that of the laser material inoptical alignment with the laser material. This compensating material isthen artificially heated to thermally induce artificial stress in thematerial in a sense opposite to thermal stress developed in the lasermaterial. The degree of heating. is changed until the birefringenceartificially induced in the compensating material substantially cancelsthe birefringence induced by thermal stress in the laser material. v

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of theinvention will be had by referring to the accompanying drawings inwhich:

FIG. 1 is a schematic illustration of a prior art solid state lasersystem with which the present invention may be used;

FIG. 2 is an end view in the direction of the arrows 2-2 of the lasermaterial utilized in FIG. 1 illustrating birefringence induced in thematerial;

FIG. 3 is a qualitative plot of temperature at different radialdistances within the laser material of FIG. 2;

FIG. 4 is a diagrammatic front view of a compensating system forcompensating the birefringence described in FIGS. 2 and 3;

FIG. 5 is a cross section taken in the direction of the arrows 5-5 ofFIG. 4;

FIG. 6 illustrates the temperature at different radial distances withinthe compensating material of FIGS. 4 and 5; and

FIG. 7 is a diagrammatic showing of the laser system of FIG. 1incorporating the compensating system of FIGS. 4 and 5.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1 there is showna laser material in the form of a rod 10 surrounded by an helicalflashlamp ll poweredfrom a light pump source 12. End mirrors l3 and 14in optical alignment with the rod 10 define an optical cavity for thelaser. Light pumping of the rod 10 by the flashlamp ll builds up aninverted population level of the laser ions in the rod and at threshold,the laser ions in the upper energy level fall back to a lower levelresulting in stimulated emission of radiation which is enhanced throughregenerative action taking place between the end mirrors l3 and 14. Theend mirror 14 may be partially transmissive in order to couple the beamout of the optical cavity.

Normally, the laser rod 10 and helical flashlamp 11 are surrounded by anenclosure or "head" through which cooling liquid is circulated. Coolingof the rod 10 is thus effected essentially at the cylindrical surfaceand a temperature gradient is developed in the rod from the centertowards the periphery.

With particular reference to FIG. 2, the temperature of the rod 10 atits center is designated T and is substantially greater than thetemperature of the rod designated T, at its surface. This temperaturegradient generates thermal stress such that the center of the rod isessentially under compression and the surface under tension.

A light beam incident at a point P is broken up into two rays, polarizedin radial and tangential directions respectively. The refractiveindexes, respectively designated n, and n,, are different in bothdirections. In the case of a neodymium doped YAG laser rod, therefractive index for the radial component of the polarized light islarger than for the tangential component. This relationship isillustrated by the index ellipse shown in FIG. 2.

FIG. 3 illustrates. at 15 the variation in temperature described for thelaser material 10 of FIG. 2. It will be noted that the temperature ismaximum at the center of the rod and decreases with increasing radiustowards the periphery.

In accord with the method and means of the present invention, there isintroduced into the laser cavity of FIG. I a compensating material whichhas physical properties similar to the laser material; for example, aradial symmetry which is the same as the laser rod but in which for theparticular example illustrated there is induced a refractive index forthe tangential component which is larger than that for the radialcomponent.

By providing such a compensating material in optical alignment with thelaser rod in accord with the method of the invention, birefringencedeveloped in the laser rod is essentially canceled by the artificiallyinduced birefringence in the compensating material.

Referring to FIG. 4, there is illustrated one practical means ofeffecting the above-described compensation. As shown, the compensatingmaterial'is in the form of a disc 16 of diameter at least equal to thatof the laser rod 10 of FIG. 2. This disc has similar physical propertiesto that of the laser rod and, for example, in the case of a YAG laser,the disc 16 itself would preferably consitute undoped YAG.

As shown in FIG. 4, a ring of metal 17 surrounds the periphery of thedisc and is in intimate thermal contact therewith. Mounted on the ring17 are electricalheating elements such as indicated at l8. The degree ofheat applied by these elements may be adjusted or changed by a suitablecontrol means such as an electrical control designated by the box 19which varies the current supplied to the heating elements.

FIG. 5 depicts the arrangement of the disc, metallic ring, and heatingelements in cross section.

With the foregoing arrangement, the application of heat to the elementswill heat the ring and thus induce a temperature gradient in the discmaterial 16 wherein the temperature at the outer surface is maximum andthe temperature at the center minimum. This type of thermal gradient isillustrated in FIG. 6 at 20 wherein it will be noted that thetemperature increases from the center radially outwardly. The indexellipse is shown in FIG. 4 wherein the tangential component n, is newlarger than the radial component n,'.

FIG. 7 illustrates the compensating disc and associated componentswithin the laser cavity of the system of FIG. 1 wherein the samereference numerals have been used.

OPERATION The operation of the compensation means will be evident fromthe foregoing description. In thecase of any particular solid statelaser rod, the compensating disc is preferably formed of a materialcorresponding to the host crystal material of the laser. For example,such material may constitute glass, or in the case of a YAG rod, undopedyttrium-aluminum garnet.

The disc is disposed in the optical cavity in alignment with the laserrod as illustrated in FIG. 7 and heat is then applied to the peripheralring 17 by controlling the current supplied to the heating elements 18from the control 19. in

In the particular example set forth, the artificial thermal gradientinduced in the compensating disc extends from the periphery towards thecenter which is opposite in sense to the thermal gradient developed inthe'laser rod when peripheral cooling is employed. Artificial thermalstresses are thus established in the compensating disc which in turninduce a birefringence in the disc.

With the compensating disc in the optical cavity, the degree of heatapplied to the compensating disc may be varied during operation of thelaser to change the thermal stress and thus artificially inducebirefringence until cancellation of the birefringence in the lasermaterial occurs.

It will be understood that the output from many diflerent laser rods isalready in the form of linear polarized light. In

other systems there would be provided a polarizing plate in the opticalcavity of the system of FIG. 7 to provide linear polarized light forelectro-optical Q switching or other opera tions requiring a polarizedbeam.

1 By cancelling the birefringence for such linear polarized light,increased power output is realizable over that obtainable in the absenceof the compensating material.

lclaim:

l. A method of compensating birefringence induced by thermal stress in alaser material, comprising the steps of:

a. introducing a compensating material of physical properties similar tothat of said laser material in optical alignment with said lasermaterial;

b. artificially heating said compensating material to thermally induceartificial stress in said material in a sense opposite to thermal stressdeveloped in said laser material; and

c. changing the degree of heating of said compensating material untilthe birefringence artificially induced by said artificial stresssubstantially cancels the birefringence induced by thermal stress insaid laser material. 2. Means for compensating birefringence induced bythermal stress in a laser material, comprising:

a. compensating material of physical properties similar to that of saidlaser material;

b. heating means for applying heat to said compensating material atpoints to establish thermal gradients in said material in directions toinduce artificial stress in a sense opposite to thermal stress developedin said laser material; and

c. means for changing the heat applied by said heating means until thebirefringence artificially induced by said artificial thermal stresscancels the birefringence induced by thermal stress in said lasermaterial when said compensating material is disposed within an opticalcavity incorporating said laser material and is in optical alignment Iwith said laser material whereby increased power output from said lasermaterial is provided over that obtained in the absence of saidcompensating material.

3. The subject matter of claim 2, in which said compensating material isin the form of a disc member, 'said heating means including a thermallyconducting ring of metal in intimate thermally conductive contact withthe periphery of said disc member; and electrical heating elementsmounted on said ring for heating said ring to thereby establish radialinwardly directed thermal gradients; and in which said means forchanging the heat applied includes electrical control means connected tosaid electrical heating elements for varying the current suppliedthereto.

4. A laser system comprising, in combination:

a. a laser material;

b. light pumping means for inducing an inverted population level oflaser ions in said material;

c. first and second end mirrors positioned in optical alignment withsaid material to define an optical cavity;

d. a compensating material of physical properties similar to that ofsaid laser material disposed in saidoptical cavity in optical alignmentwith said laser material;

e. artificial heating means connected to said compensating material atpositions to establish thermal gradients in said compensating materialin directions to induce artificial thermal stress in a sense opposite tothermal stress developed in said laser material by said light pumpingmeans; and

f. control means connected to said heating means for changing the degreeof heat and thereby the birefringence induced by said artificial thermalstress in said compensating material until it substantially cancels thethermally induced birefringence in said laser material.

5. A system according to claim 4, in which said laser material comprisesa solid state rod of given diameter, said compensating material being inthe form of a disc member of diameter at least equal to said givendiameter, said heating means including a thermally conducting ring ofmetal in intimate thermally conductive contact with the periphery ofsaid disc member; and electrical heating elements mounted on said ringfor heating said ring to thereby establish radial inward directions forsaid thermal gradients; and in which said control means includeselectrical means connected to said electrical heating elements forvarying the current supplied thereto.

6. A system according to claim 5, in which said disc material comprisesglass.

7. A system according to claim 5, in which said rod comprises ayttrium-aluminum-gamet host crystal doped with neodymium ions and inwhich said disc member comprises un- I doped yttrium-aluminum-gamet.

# t it

1. A method of compensating birefringence induced by thermal stress in alaser material, comprising the steps of: a. introducing a compensatingmaterial of physical properties similar to that of said laser materialin optical alignment with said laser material; b. artificially heatingsaid compensating material to thermally induce artificial stress in saidmaterial in a sense opposite to thermal stress developed in said lasermaterial; and c. changing the degree of heating of said compensatingmaterial until the birefringence artificially induced by said artificialstress substantially cancels the birefringence induced by thermal stressin said laser material.
 2. Means for compensating birefringence inducedby thermal stress in a laser material, comprising: a. compensatingmaterial of physical properties similar to that of said laser material;b. heating means for applying heat to said compensating material atpoints to establish thermal gradients in said material in directions toinduce artificial stress in a sense opposite to thermal stress developedin said laser material; and c. means for changing the heat applied bysaid heating means until the birefringence artificially induced by saidartificial thermal stress cancels the birefringence induced by thermalstress in said laser material when said compensating material isdisposed within an optical cavity incorporating said laser material andis in optical alignment with said laser material whereby increased poweroutput from said laser material is provided over that obtained in theabsence of said compensating material.
 3. The subject matter of claim 2,in which said compensating material is in the form of a disc member,said heating means including a thermally conducting ring of metal inintimate thermally conductive contact with the periphery of said discmember; and electrical heating elements mounted on said ring for heatingsaid ring to thereby establish radial inwardly directed thermalgradients; and in wHich said means for changing the heat appliedincludes electrical control means connected to said electrical heatingelements for varying the current supplied thereto.
 4. A laser systemcomprising, in combination: a. a laser material; b. light pumping meansfor inducing an inverted population level of laser ions in saidmaterial; c. first and second end mirrors positioned in opticalalignment with said material to define an optical cavity; d. acompensating material of physical properties similar to that of saidlaser material disposed in said optical cavity in optical alignment withsaid laser material; e. artificial heating means connected to saidcompensating material at positions to establish thermal gradients insaid compensating material in directions to induce artificial thermalstress in a sense opposite to thermal stress developed in said lasermaterial by said light pumping means; and f. control means connected tosaid heating means for changing the degree of heat and thereby thebirefringence induced by said artificial thermal stress in saidcompensating material until it substantially cancels the thermallyinduced birefringence in said laser material.
 5. A system according toclaim 4, in which said laser material comprises a solid state rod ofgiven diameter, said compensating material being in the form of a discmember of diameter at least equal to said given diameter, said heatingmeans including a thermally conducting ring of metal in intimatethermally conductive contact with the periphery of said disc member; andelectrical heating elements mounted on said ring for heating said ringto thereby establish radial inward directions for said thermalgradients; and in which said control means includes electrical meansconnected to said electrical heating elements for varying the currentsupplied thereto.
 6. A system according to claim 5, in which said discmaterial comprises glass.
 7. A system according to claim 5, in whichsaid rod comprises a yttrium-aluminum-garnet host crystal doped withneodymium ions and in which said disc member comprises undopedyttrium-aluminum-garnet.